If you did spell something wrong, then a red line appears underneath to notify you of a word that is wrong. No red line, no spelling mistakes. If you do get a red line, right click for a list of alternatives.
Though you must understand that a correct, out-of context spelling will not be flagged
i.e. To rip presents you need paper and sticky type (rip should be wrap and type should be tape, but because they are the correct spelling, they are not flagged)